Distributed process control systems, like those used in chemical, petroleum or other process plants, typically include one or more process controllers communicatively coupled to one or more field devices via analog, digital or combined analog/digital buses, or via a wireless communication link or network. The field devices, which may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure, level and flow rate sensors), are located within the process environment and generally perform physical or process control functions such as opening or closing valves, measuring process parameters, etc. to control one or more processes executing within the process plant or system. Smart field devices, such as the field devices conforming to the well-known Fieldbus protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within the controller. The process controllers, which are also typically located within the plant environment, receive signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices and execute a controller application that runs, for example, different control modules which make process control decisions, generate control signals based on the received information and coordinate with the control modules or blocks being performed in the field devices, such as HART®, Wireless HART®, and FOUNDATION® Fieldbus field devices. The control modules in the controller send the control signals over the communication lines or links to the field devices to thereby control the operation of at least a portion of the process plant or system.
Information from the field devices and the controller is usually made available over a communications backbone to one or more other hardware devices, such as operator workstations, personal computers or computing devices, data historians, report generators, centralized databases, or other centralized administrative computing devices that are typically placed in control rooms or other locations away from the harsher plant environment. Each of these hardware devices typically is centralized across the process plant or across a portion of the process plant. These hardware devices run applications that may, for example, enable an operator to perform functions with respect to controlling a process and/or operating the process plant, such as changing settings of the process control routine, modifying the operation of the control modules within the controllers or the field devices, viewing the current state of the process, viewing alarms generated by field devices and controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, keeping and updating a configuration database, etc. The communications backbone utilized by the hardware devices, controllers and field devices may include a wired communication path, a wireless communication path, or a combination of wired and wireless communication paths.
As an example, the DeltaV™ control system, sold by Fisher-Rosemount Systems, Inc., includes multiple applications stored within and executed by different devices located at diverse places within a process plant. A configuration application, which resides in one or more workstations or computing devices, enables users to create or change process control modules and download these process control modules via a communications backbone to dedicated distributed controllers. Typically, these control modules are made up of communicatively interconnected function blocks, which are objects in an object oriented programming protocol that perform functions within the control scheme based on inputs thereto and that provide outputs to other function blocks within the control scheme. The configuration application may also allow a process engineer to create or change operator interfaces which are used by a viewing application to display data to an operator and to enable the operator to change settings, such as set points, within the process control routines. Each dedicated controller and, in some cases, one or more field devices, stores and executes a respective controller application that runs the control modules assigned and downloaded thereto to implement actual process control functionality. The viewing applications, which may be executed on one or more operator workstations (or on one or more remote computing devices in communicative connection with the operator workstations and the communications backbone), receive data from the controller application via the communications backbone and display this data to process control system designers, operators, or users using the user interfaces, and may provide any of a number of different views, such as an operator's view, an engineer's view, a technician's view, etc. A data historian application is typically stored in and executed by a data historian device that collects and stores some or all of the data provided across the communications backbone while a configuration database application may run in a still further computer attached to the communications backbone to store the current process control routine configuration and data associated therewith. Alternatively, the configuration database may be located in the same workstation as the configuration application.
In many processes, legacy Programmable Logic Controllers (PLCs) are integrated into the process. For instance, older portions of a process plant may originally have implemented PLCs to control those portions of the process plant. As the process plant may have expanded or modernized, portions of the process plant may have implemented non-PLC distributed control solutions (e.g., the aforementioned DeltaV™ control system), while leaving in place the legacy PLC solutions that were already implemented and choosing to integrate the legacy PLCs into the new systems. For a variety of reasons, integrating the legacy PLCs into the new systems—usually with significant investments of time and effort—remained a preferable solution.
One impediment to retrofitting legacy PLC-based plant configurations is legacy wiring. Facilitating communication between the field devices in the process plant and the devices—such as PLCs and controllers—that implement control strategies to operate those field devices, typically involves running wiring from the field devices to a marshaling cabinet or other centralized area at which the wires are organized and terminated. Typically, input signals from the field devices to the control system are brought together into groups, while output signals from the control system to the field devices are grouped together. The groups of wires (and the signals they carry) may be further grouped by the types of signals carried by the wiring (e.g., by voltage, according to whether the signals are discrete or analog, etc.), and terminated at or near a set of I/O interface devices. The I/O interface devices facilitate communication of the signals to the control device.
In some systems, for example, each group of wires may be terminated at an associated swing-arm or I/O card field termination connector that couples the signal carried on the wires to a corresponding I/O card by means of a connector—such as a card-edge connector, a contact connector, etc.—that typically has a higher signal density than the wiring itself. The use of the swing-arm allows the wiring to be disconnected from the I/O card for maintenance or trouble-shooting purposes. That is, by removing the swing-arm from the I/O card, the I/O card can be removed and replaced if it fails, without having to individually detach (and later re-terminate) each of the numerous wires that the swing-arm carries.
While the use of swing-arms facilitates movement of the wiring bundle, that movement is limited by a variety of factors. For example, as can be appreciated, large bundles of wiring can be difficult to manipulate due to the relatively large amounts of copper contained therein. Adding to this difficulty is the fact that legacy wiring, in particular, may have become less flexible over time, or may have insulation that has become brittle, putting the wiring at risk for short circuits.
Another impediment to retrofitting legacy PLC-based plant configurations is consideration of available space. Typically, process plants (or portions thereof) do not leave significant room for later expansion and, as a result, it can be difficult to perform a retrofit without physical modification of the process plant (or portion thereof), extended disruption of the process (and associated revenue), and the like. The newer control systems typically require space that is not available in the rack rooms that house the legacy PLC-based solutions, and sending the signals from the rack rooms housing the terminated signals to the controllers in other available space can require many long lengths of cable running through small spaces, which can result in signal cross-talk, the result of which is unreliability of the process as a whole.